Potassium titanyl phosphate (KTiOPO.sub.4) has long been recognized as an outstanding material for many important optical and electro-optical applications. The basic element of optical communications systems is the optical waveguide which transmits or guides optical waves from one point to another and serves to connect various devices in an optical integrated circuit.
U.S. Pat. No. 4,766,954 of Bierlein et al., issued Aug. 30, 1988, discloses an ion exchange process for producing an optical waveguide by contacting a single crystal of K.sub.1-x Rb.sub.x TiOMO.sub.4 wherein x is 0 to 1 and M is P or As with a molten salt of Rb, Cs or Tl for a time sufficient to increase the surface index of refraction with respect to that of the starting crystal.
U.S. Pat. No. 4,740,265 of Bierlein et al., issued Apr. 26, 1988, claims the resulting waveguide and its use in an optical waveguide device prepared by the process of the above patent.
Bierlein et al., Appl. Phys. Lett., Vol. 50, No. 18, pp. 1216-1218, May 4, 1987, discloses the fabrication and characterization of planar and channel optical waveguides in KTiOPO.sub.4 using ion exchange processes.
U.S. Pat. No. 3,998,687 of Ballman et al., issued Dec. 21, 1976, discloses a liquid phase epitaxial growth technique for optical waveguides comprising lithium niobate films on lithium tantalate substrates using particular flux systems.
Bierlein et al., Appl. Phys. Lett., Vol. 54, No. 9, pp. 783-785, Feb. 27, 1989, discusses the linear and nonlinear optical properties of flux-grown KTiOAsO.sub.4.
Morris et al., Mat. Res. Soc. Symp. Proc., Vol. 2, pp. 95-101 (1989) discusses defects in the nonlinear optical crystal KTiOPO.sub.4 and relates its properties of ionic conductivity and damage susceptibility to the specific technique and conditions used for growth.
Waveguides in KTiOPO.sub.4 consist primarily of the cationic solid solution K.sub.x M.sub.1-x TiOPO.sub.4, wherein M can be rubidium, cesium, or thallium, formed mainly by an ionexchange process. The ionic conductivity of KTiOPO.sub.4 varies significantly with crystal growth method and impurities, making the device fabrication process difficult and with poor yields. This broad variability in the ionic conductivity is problematic and can limit the development of highly efficient waveguide frequency doubling devices. Due to the inherently diffusive nature of the ion exchange process, ion-exchanged waveguides generally exhibit a diffused refractive index profile along one axis which is believed to be responsible for variation in the performance of these devices. This type of index profile, though satisfactory for many applications, is less effective in the confinement of optical fields, which in turn makes the devices less useful in applications such as waveguide second harmonic generation.
It is therefore an object of the present invention to provide an optical waveguide with a well defined step-like refractive index profile.
It is a further object of the present invention to provide an optical waveguide in which the diffusiveness due to conductivity does not significantly affect its waveguiding properties.
It is a further object of the present invention to provide a liquid phase epitaxy process for the preparation of waveguides having the above properties.